In recent years, various types of hybrid vehicles having an internal combustion engine and an electric motor as drive sources have been developed. Such a hybrid vehicle uses the internal combustion engine and the electric motor in combination according to the driving state of the vehicle, thereby improving the fuel consumption rate and the emission performance, while ensuring a sufficient dynamic performance of the vehicle.
The internal combustion engine is driven by being supplied with fuel in the fuel tank, and the electric motor is driven by being supplied with electricity of the electric storage device. To maintain the state of charge of the electric storage device in a predetermined range, the electric storage device is charged by electricity supplied by a generator driven by the internal combustion engine when the state of charge is low. A type of a hybrid vehicle has been developed in which such an electric storage device can be charged by electricity supplied from the outside of the vehicle (for example, refer to Japanese Laid-Open Patent Publication No. 2007-62638). Such a hybrid vehicle reduces the load on the internal combustion engine required for driving the generator. Thus, the fuel consumption rate and the emission performance are further improved.
In such a hybrid vehicle, the load on the internal combustion engine is reduced when the state of charge of the battery charger is in a favorable range. Thus, the fuel in the fuel tank is likely to remain unused for a long period and thus become degraded. This degrades the startability and the combustion state of the internal combustion engine. Particularly, in a vehicle disclosed in Japanese Laid-Open Patent Publication No. 2007-62638, in which the electric storage device can be charged by electricity supplied from the outside of the vehicle, the state of charge of the electric storage device is maintained in a predetermined range due to a continuous charging. As a result, the internal combustion engine can remain unused for a long period. This promotes the degradation of the fuel in the fuel tank.